Before Sullivan Square was a tangle of speeding cars, industrial properties, and parking lots, it was a proper city square, with residences and businesses framing a large central park. But the park is long gone, swallowed whole by the train tracks and roadway lanes that now define the square. A neighborhood defined by the infrastructure slicing through it stopped functioning as a neighborhood at all.

Bad planning broke Sullivan Square, but good planning can fix it. Boston is at the cusp of turning the square into a destination, rather than just a place that commuters cut through when traffic backs up on Interstate 93. If the turnaround is going to stick, though, public officials will have to make sure that the very thing that enables Sullivan Square’s transformation — the subway running through it — doesn’t become its undoing.

The square today is little more than a large rotary and two-level roadway connecting Everett and Somerville to Boston. These roads are now on their way out — the Big Dig made the wide, depressed Route 99 arteries that cut through Charlestown irrelevant, while a structurally unsound overpass that once soared above the square was demolished a decade ago. Boston is now pushing ahead with a plan to bring the depressed artery to the surface, slice it in half, and line it with trees and bike paths. The downsizing project will eliminate the massive rotary, and free up acres of asphalt.

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Late last month, city officials unveiled a plan to couple this road-slimming project with an effort to rebuild Sullivan Square. They identified seven parcels around the rebuilt Route 99 and the Sullivan MBTA station for redevelopment. The plan envisions filling these parcels — they’re now surface parking lots, traffic lanes, or low-grade industrial properties — with blocks of residences, offices, shops, and hotel rooms. The end result would be a walkable Sullivan Square, one that both improves access to the T from the surrounding neighborhood and creates a new gateway to Somerville, Cambridge, and Everett.

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The proposed reinvention gets city-building right. It values sidewalks over oversized roadways. It embraces dense development to create new centers of activity. And it leverages existing Orange Line transit to turn cheap surface parking lots into housing and high-value commercial space. The vision for Sullivan Square looks a lot like what’s happening down the Orange Line at Jackson Square, at NorthPoint in Cambridge, and nearby, at Somerville’s Assembly Square, Inner Belt, and Union Square.

The Sullivan project has a leg up over most of these others: Since most of the land around the station is publicly owned, Boston can control both the timing of the development and the quality of the finished product.

The city plan cautions that this vision for Sullivan Square is years away, since, it argues, development economics wouldn’t support much of it today. The economics aren’t the real danger. With direct transit access, the downtown just a quick Orange Line ride away, and the potential to use cheap public land to subsidize new affordable housing, Sullivan Square is the perfect place to go big on housing construction. The nearby Schrafft’s building is poised to become a hub for small, innovative businesses.

The real danger is the subway line running through the square — the very thing that makes Sullivan’s transformation possible in the first place. The T has to be able to handle robust development around Sullivan station, and, right now, it can’t.

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Even without thousands of new commuters living around Sullivan and Assembly Square, on most days, the Orange Line is over capacity. The T’s perpetual budget battles have starved basic maintenance for years, and there’s no end in sight. Last year’s transportation funding package fixed the T’s operating deficits, but didn’t do anything to put more cars on the line. The T’s own budget forecast huge maintenance needs in coming years, but make up for those costs with huge increases in ridership. Those new riders can’t come if budget woes mean they can’t fit on a train in the morning. And if new riders can’t fit on the train, then all the good planning in the world won’t turn the massive MBTA-owned parking lot outside Sullivan station into something else.

Paul McMorrow is an associate editor at Commonwealth Magazine. His column appears regularly in the Globe.